The present invention is directed to a diaphragm pressure sensor and a method of fabricating the same, and in particular to a diaphragm pressure sensor for sensing a fluid pressure in, for example, a container for chemicals, a pipe for chemicals or the like, and a method of fabricating the same.
Conventional pressure sensors for sensing a fluid pressure in a container for chemicals, a pipe for chemicals or the like, are generally provided with a diaphragm which acts as a pressure-sensing means, whereby deflection of the diaphragm in response to an applied pressure is translated into an electric signal, to thereby sense a pressure.
Japanese Patent Application No. 2002-130442 discloses an example of such a diaphragm pressure sensor in the invention titled “Electrical capacitance diaphragm pressure sensor”.
Such a diaphragm pressure sensor comprises, for example, :a pressure-sensing element provided with a pressure receiving part including strip-shaped or rectangular flat plate-shaped diaphragms provided in opposing relation, and deposition electrodes formed on opposing surfaces of the diaphragms; a housing element for enclosing the pressure receiving part of the pressure-sensing element, the housing element being made of a material which is resistant to corrosion by a fluid whose pressure is to be detected by the sensor; and an electronic circuit for detecting deflection of the diaphragms.
Such a diaphragm pressure sensor as described above is constituted such that when immersing a housing element in a fluid whose pressure is to be measured, the fluid pressure acts on a pressure receiving part, and any resulting variations in distance between opposing diaphragms cause a change in capacitance.
In a conventional diaphragm pressure sensor such as that described above, a pressure transfer coefficient varies according to a temperature of a fluid whose pressure is to be measured, and instability such as temperature drift and the like is thereby caused, and as a result, measurement accuracy is significantly compromised. It is known that a leading cause of temperature drift in a diaphragm pressure sensor is a thermal expansion/contraction coefficient of a diaphragm material.
With a view to preventing temperature drift from disadvantageously affecting measurement by a diaphragm pressure sensor, a conventional diaphragm pressure sensor, especially a metal diaphragm pressure sensor, employs a temperature compensation circuit in a pressure sensing circuit for sensing a pressure deflection of a diaphragm, or disposes a temperature sensor in a diaphragm to measure a temperature of the diaphragm and provide a compensation electric signal commensurate with the thus measured temperature to a pressure sensing circuit, to thereby compensate for temperature drift, that is, a thermal expansion/contraction coefficient of a diaphragm material in accordance with a temperature.
As a pressure-sensing element, a sapphire diaphragm pressure sensor in which a diaphragm is made of a sapphire plate, and a ceramic diaphragm pressure sensor in which a diaphragm is made of an alumina ceramic plate, are also known. Since sapphire and alumina ceramic have a considerably smaller thermal expansion coefficient compared to metallic materials, they can compensate for temperature drift effectively. However, both a sapphire, which is a crystallization of alumina, and an alumina ceramic, which is made of a sintered body of alumina, gradually erode when they come into contact with a strong acid fluid such as a highly concentrated fluoric acid solution or nitrate solution, and therefore, they are not desirable in terms of corrosion resistance.
Following are a few conventional ways to impart corrosion resistance to a sapphire diaphragm, a ceramic diaphragm or the like.    (1) A fluororesin is applied on the surface of a diaphragm to form a fluororesin coating and thereby improve corrosion resistance.    (2) A relatively thick diaphragm of fluororesin is formed, upon which another diaphragm made of a sapphire plate, a ceramic plate or the like is overlaid to fabricate a double diaphragm and thereby improve corrosion resistance.
However, improvement measures such as those described above still cannot solve the following problems:    (1) A fluororesin per se significantly expands and shrinks with temperature, which causes stress strain on a diaphragm and temperature drift in a diaphragm pressure sensor.    (2) When a fluororesin is simply applied to the surface of a diaphragm to form a fluororesin coating, such a coating cannot be tightly secured to the diaphragm, and easily peels.    (3) As a fluororesin per se is non-adherent, when preparing a double diaphragm, an adhesive containing an amine or the like is employed to bond a fluororesin diaphragm to a diaphragm made of a sapphire plate, a ceramic plate or the like. However, a relatively great thickness of a fluororesin diaphragm, and lack of uniform thickness of an adhesive over a diaphragm, sometimes prevents a pressure from being thoroughly communicated between the fluororesin diaphragm and the diaphragm made of a sapphire plate, a ceramic plate or the like.    (4) In a double diaphragm, a relatively thick fluororesin diaphragm allows temperature drift to become great due to thermal expansion/contraction of the fluororesin diaphragm per se. Therefore, a pressure sensor employing such a double diaphragm can be used only under certain temperature conditions.